1. Field of Invention
The present invention relates to thermal spray processes and, more particularly, to a composite electrode wire construction for use in an electric arc thermal spray process.
2. Description of the Related Prior Art
A thermal sprayed coating is a surface layer that comprises a protective coating to a substrate which is generally highly resistant to wear, erosion, abrasion and corrosion. In an electric arc thermal spraying process, a DC electric arc is used to directly melt consumable electrode wires. Specifically, in twin wire-arc spraying, the electric current is carried by two electrically conductive, consumable wires. An electric arc is created between the wire tips across a gap created by the continuous convergence of the two wires. A gas jet blows from behind the converging electrodes and transports the molten metal that continuously forms as the wires are melted by the electric arc. The gas jet breaks up or atomizes the molten metal and directs it toward a substrate. As the sprayed particles impinge on the substrate, they cool and accumulate into a hard coating on the surface of the substrate. The gas jet may comprise air, oxygen, argon, or other gases, depending on the requirements of the application.
In the past, electrode wires formed of a combination of nickel and aluminum have commonly been used to provide the thermal spray material, where the weight percentage of nickel is substantially larger than the weight percentage of aluminum. In particular, the aluminum functions to act as a fluxing material for cleaning the substrate surface as well as a bonding material which forms a surface layer on the substrate to which the hard nickel material can adhere. The material forming the thermal spray coating is known as a “bondcoat” material, and generally includes a minimum of 5% aluminum with the balance of the material being nickel. A common composition for nickel/aluminum electrode wires comprises approximately 85% nickel and 15% aluminum, and the electrode wires are formed in a variety of constructions including solid wires comprising a combination of nickel and aluminum, or as tubular wires comprising an outer tube of nickel containing either a solid aluminum wire or an aluminum powder. Solid wire electrodes are generally more expensive and have lower productivity than the tubular electrode wire constructions. However, known tubular wire electrodes, although less expensive, also suffer from problems resulting in process inefficiencies. For example, it has been found that electrodes formed with a nickel tube containing an aluminum powder have the disadvantage that the aluminum powder is more volatile than solid aluminum and the particles forming the powder provide a larger surface area resulting in the powder interfering with the thermal efficiency of the spray process, which retards the efficiency of the thermal spray coating application. The tubular electrodes having a solid aluminum wire core require that the solid aluminum wire be fit in tight contact within the surrounding nickel tube, in that separations between the core and the outer surface of the electrode will result in discontinuities in the electrical current supplied to the electrode tip and associated inconsistencies in melting of the electrodes.
Accordingly, there is a need for a thermal spray electrode which ensures consistent application of a thermal spray coating and which facilitates the productivity of the thermal spray operation.